Conversion coatings are widely used to treat metal surfaces to improve corrosion resistance, increase adhesion of subsequent coatings such as paint, form a decorative finish, or retain electrical conductivity. Conversion coatings are formed by applying a conversion coating solution to the metal. The conversion coating solution and the metal react to convert or modify the metal surface into a thin film with the desired functional characteristics. Conversion coatings are particularly useful for the surface treatment of metals such as aluminum, zinc, and magnesium.
Conversion coatings containing hexavalent chromium have been used for years to successfully treat a variety of metal surfaces. Despite their success, they have fallen out of favor due to the high toxicity of hexavalent chromium. This compound significantly increases the hazards for process personnel and increases operating and waste disposal costs, particularly in locations where it is highly regulated and subject to burdensome use and handling restrictions. Hexavalent chromium conversion coatings are being replaced by non-hexavalent chromium conversion coatings such as those utilizing trivalent chromium.
Dyes can be added to convention hexavalent conversion coatings to impart a specific color to the metal surface. This may be done to make a part easy to visually identify and/or otherwise give it a unique visual appearance. The use of dyes with naturally clear trivalent chromium conversion coatings has enjoyed less success. There are few dyes that are suitable for use with trivalent chromium conversion coatings.
A few trivalent chromium conversion coating solutions have been formulated over the years with a dye. One such solution includes copper phthalocyanine as the dye and is used to coat aluminum. Another solution includes catechol violet as the dye and is also used to coat aluminum. Although these conversion coating solutions work to a certain extent, they suffer from a number of problems.
One problem is that they tend to produce weak and uneven color on the metal surface, which make them unsuitable from an aesthetic standpoint. Another problem is that they are incompatible with many trivalent chromium conversion solutions. The dyes can cause substantial precipitation of the constituents in the trivalent chromium conversion coating solution. The precipitation causes powdering of the surface of the metal, deactivation of the bath, reduction in coating performance—specifically corrosion resistance, and the need for continual replenishment of the dye. In some situations, the dyes can quickly render the solution completely inactive. This is especially a problem with trivalent chromium coating solutions containing phosphates.